1. Field of the Invention
The present invention generally relates to a drum cleaning unit for mechanically removing residual toner from a circumferential surface of a photoconductive drum after the transfer of a toner image in an image recording apparatus, and specifically to units used in such apparatus utilizing electrophotographic image transfer systems.
2. Discussion of Background Invention
Conventional image recording devices utilizing electrophotographic transfer systems are known in which the surface of a photoconductive drum is exposed to light to form a latent image on the drum surface, toner is then applied to the latent image to develop the image, and the developed image is transferred onto a recording sheet, where it is then fixed by a fixing unit. Such image recording devices are chiefly employed in copying machines. In recent years, however, such image recording devices have also been utilized in printers and like apparatus for printing the outputs of computers; one such apparatus is the laser beam printer.
The laser beam printer comprises, as in one example which is illustrated in FIG. 1, a photoconductive drum 1. Arranged about the photoconductive drum 1, in sequential order as viewed in the rotational direction of the drum, are a charging station A, an exposure station B, a developing station C, a transferring station D, a toner-cleaning station E, and a discharge station F.
The arrangement is such that at the exposure station B, a laser beam scans the surface of the drum 1, which has been uniformly charged at the charging station A, to thereby form a latent image on the charged drum surface. Toner is then applied at the developing station c to the latent image in order to develop the same. Subsequently, the developed toner image is transferred at the transferring station D onto the recording sheet P, which is traveling at a velocity which is identical to the peripheral speed of the photoconductive drum 1. The recording sheet P, carrying the toner image transferred onto the sheet at the transfer station D, is guided and/or fed by guide rollers to a fixing station G. The recording sheet P is then heated and/or pressed at the fixing station G so that the toner image will be fixed onto the surface of the recording sheet P.
Drum cleaning methods broadly fall into the following two types: the first is the "blade method", using a blade for scraping residual toner from the circumferential surface of the photoconductive drum by pressing the blade against the photoconductive drum under a given pressure; and the second is the "brush method", comprising the steps of charging a rotary brush formed of chargeable material, making it both rotate around and contact the circumferential surface of the photoconductive drum, having a residual toner on the drum electrostatically absorbed by the brush, and then letting toner mechanically fall from the brush.
The blade method is generally preferred to the brush method because the latter tends to make construction of the cleaning system complicated as well as costly.
The blade used in the blade method is generally made of an elastic polymeric compound, such as synthetic or natural rubber, and should be capable of completely removing residual toner without wearing and damaging the photoconductive surface of the drum.
If, however, such a blade is made of hard material, it will quickly wear and/or damage the photoconductive drum surface, because the drum surface is relatively poor in mechanical strength (i.e., particularly in its wear resistance). Such undue and undesirable wear often occurs because the blade is made of the aforementioned elastic material, in the form of a flat plate having a given thickness, such that the blade will continually touch the surface of the photoconductive drum.
As a result, the cleaning capability and efficiency of the blade method is generally dependent upon various conditions, including the hardness and shape (i.e., the length and thickness) of the blade, as well as the contact angle between the blade and the circumferential surface of the photoconductive drum.
There are generally two ways to contact the blade with the surface of the photoconductive drum; one is the so-called "trailing way", in which the blade is inclined towards the upstream side of the rotation of the photoconductive drum, with respect to a line which is perpendicular to a tangent line at the contact point; the second way is the so-called "counter way", in which the blade is inclined, contrary to the first way, towards the downstream side of the rotation of the photoconductive drum.
In any event, the resultant force of the reaction force generated when the blade is pressed against the photoconductive drum, and the frictional force generated as the photoconductive drum rotates, is applied to the blade. When the trailing way is used, the resultant force urges the blade to bend in a direction in which the blade is directed away from the photoconductive drum 1, whereby the contact pressure is weakened and the photoconductive drum is insufficiently cleaned. Accordingly, in such cases the blade must have a given hardness and thickness, for instance, a hardness H, as determined by the JIS (i.e., the Japanese Industrial Standard) Scale A, equal to 70, and a thickness T=3 mm. Further, such a blade should be pressed against the surface of the drum under relatively strong pressure, in order to insure adequate cleaning of the drum.
Moreover, since the blade has to be pressed against the whole corresponding longitudinal area on the circumferential surface of the photoconductive drum 1 under a uniform pressure, it becomes difficult to adjust/set the pressure to be exerted.
Although the contact pressure of the blade should preferably be as weak as possible (to reduce drum wear) within all possible ranges of pressure which result in a satisfactory cleaning of the drum, it tends to be set at relatively higher levels each time the pressure is adjusted, in order to sufficiently clean the circumferential surface of the photoconductive drum 1. This often results from the difficulty of appropriately setting the pressure. As a result, abrasion of the surface of the photoconductive drum is, accordingly, accelerated, so as to disadvantageously reduce the durability of the drum surface. Further, since the force or burden required to rotate the photoconductive drum increases, due to the relatively high pressure applied thereto by the blade, it becomes necessary to employ a high-power drive motor, which renders the imaging apparatus not only large-sized, but also expensive.